Powder coatings form a rapidly growing sector of the coatings market. Powder coatings are solid compositions which are generally applied by an electrostatic spray process in which the powder coating particles are electrostatically charged by the spray gun and the substrate is earthed or oppositely charged. The composition is then heated to melt and fuse the particles and to cure the coating. The powder coating particles which do not adhere to the substrate can be recovered for re-use so that powder coatings are economical in use of ingredients. Also, powder coating compositions are generally free of added solvents and, in particular, do not use organic solvents and are accordingly nonpolluting.
Powder coating compositions generally comprise a solid film-forming resin, usually with one or more colouring agents such as pigments. They are usually thermosetting, incorporating, for example, a film-forming polymer and a corresponding curing agent (which may itself be another film-forming polymer). Powder coating compositions are generally prepared by intimately mixing the ingredients, for example in an extruder, at a temperature above the softening point of the film-forming polymer(s) but below a temperature at which significant pre-reaction would occur. The extrudate is usually rolled into a flat sheet and comminuted, for example by grinding, to the desired particle size. The particle size distribution required for most commercial electrostatic spray apparatus is between 10 and 120 microns, with a mean particle size within the range of 15 to 75 microns, preferably 25-50 microns.
Whilst existing processes for the manufacture of powder coating compositions are satisfactory in some respects, there is nevertheless room for improvement and it is a general objective of the present invention to simplify the production of such compositions and also to make such production more economic.